Silicon carbide (SiC) is a material that is very stable chemically. The band gap of silicon carbide is 3 eV, which is wide enough to be extremely stably used as a semiconductor even at high temperature. In addition, because the breakdown strength of silicon carbide is also larger than that of silicon (Si) by one digit or more, attention has been paid to silicon carbide as a material alternative to silicon, whose performance limit is close to that of silicon carbide, in view of a power semiconductor device (for example, see K. Shenai and other two persons, Optimum Semiconductors for High-Power Electronics, IEEE Transactions on Electron Devices, September 1989, Vol. 36, No. 9, pp. 1811-1823).
In the last stage of processing, a wiring metal film for connecting an external device is formed in a semiconductor device. Requisite items for the wiring metal film include lots of items such as small contact resistance, non-separation at the time of dicing, and prevention of separation due to durability against long term use after bonding or die-bonding. Among these items, particularly the prevention of separation is requisite so that intensive adhesion is requisite. This is not exceptional for the case where an electrode structure of a silicon carbide semiconductor device is formed.
In a silicon carbide semiconductor device which is made of silicon carbide, silicon carbide contains carbon (C) and high temperature treatment is often used in the process of producing (manufacturing) the silicon carbide semiconductor device. For this reason, a graphite layer is apt to be formed on a surface of a silicon carbide substrate constituting the silicon carbide semiconductor device. When a metal film used for wiring etc. is deposited on the surface of the silicon carbide substrate on which the graphite layer has been generated, the metal film is apt to be separated from the silicon carbide substrate.
Particularly, when attention is paid to the step of forming an ohmic electrode serving as low resistive connection in a silicon carbide semiconductor device, it is reported that the step may be performed in such a manner that an nickel (Ni) film is deposited on a silicon carbide substrate and then subjected to heat treatment to make nickel in the Ni film and silicon in the silicon carbide substrate react with each other to thereby form, for example, an Ni silicide film on the silicon carbide substrate. However, in the case where the Ni silicide film is formed, Ni hardly reacts with carbon. Therefore, silicon in the silicon carbide substrate reacts with Ni so that carbon (hereinafter referred to as remaining carbon) whose bonding to silicon is cut off forms a graphite layer on the Ni silicide film. When a wiring metal film for making connection with an external device is formed on the graphite layer, adhesive force between the silicon carbide substrate and the wiring metal film is lowered, thereby resulting in separation of the wiring metal film.
Therefore, as a method for suppressing separation of the wiring metal film, there have been proposed a method for removing the graphite layer in a plasma process in an inert gas (argon (Ar) etc.) atmosphere, a method for forming an Ni silicide film on the Ni film before heat treatment on the Ni film to thereby prevent graphite from being deposited in the topmost surface (for example, see JP-A-2006-332358), and a method for removing deposited graphite in such a manner that a metal layer which can react with carbon is deposited on the graphite layer and then subjected to heat treatment (for example, see JP-A-2006-344688).